Antagonism of the excitatory effects of 5-hydroxytryptamine on sympathetic preganglionic neurones and neurones activated by visceral afferents.

The responses of electrophysiologically identified sympathetic preganglionic neurones (SPGN) and interneurones activated by sympathetic visceral afferents (VA), to iontophoretic application of 5-hydroxytryptamine (serotonin) and serotonin receptor antagonists: methysergide and cinanserin, were tested in the T3 segment of the spinal cord of the cat. Serotonin had a definite and dose-dependent excitatory action on sympathetic preganglionic neurones and neurones activated by visceral afferents. This excitatory response to serotonin was characterized by the rapid development of tachyphylaxis. Methysergide antagonized the effects of serotonin in preganglionic neurones and neurones activated by visceral afferents. Cinanserin was less effective than methysergide. Preliminary data on the effect of inhibitors of serotonin uptake: chloroimipramine and fluoxetine are also reported. These findings support the idea that the sympathetic output and the activity of neurones activated by sympathetic viscero-afferents is modulated by an excitatory serotoninergic supraspinal input.